Elimination of carbon in jet combustors



ELATION OF CARBON JET. COMBUSTORS John J. Kolfenbach, North Plainfield, Theodore'B wase serbach, Cranford, and Raymond W. Walker, Union, N. L, assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Application January 30, 1953, Serial N0. 334,364

Claims. (Cl. Gil-35.4) 1

The present invention relates to hydrocarbon fuel compositions, particularly for use in jet aircraft engines. It broadly relates to a method of eliminating carbonaceous deposits within internal parts of a jet engine by the introduction of an alkaline earth metal composition and an amphoteric metal composition. More particularly, it refers to hydrocarbon fuel compositions that when burned will produce combustion products that contain desired quantities of an alkaline earth metal oxide and an amphoteric .metal oxide. It alsov particularly concerns a method of eliminating carbon deposits within certain sections of a jet aircraft engine by introducing desired quantities of an alkaline earth metal oxide and an amphoteric metal oxide within said sections. Either type of oxide may be introduced as such or itmay be formed in situ within the pertinent sections of aid engine.

For the purposes of the present invention, the alkaline earth metals are considered to include barium, calcium and strontium. The amphoteric metals (e. g., the ones that are considered to form amphoteric oxides) are lead, zinc, manganese, chromium, vanadium, aluminum, bismuth, and molybdenum. The last named metal, molybdenum, forms an oxide whose amphoteric status is somewhat in question in the literature, but for the purpose of the present invention the oxide of this metal appears to be as amphoteric in its behavior as the oxides of any of the other metals named.

To best understand the present invention, it is felt desirable to first briefly describe the construction'and operation of a conventional jet aircraft engine. It will first be noted that there are three basic types of jet engines the ram-jet, the turbo-jet, and the turbo-prop. While the present invention can be applied to any one of these with the remaining or secondary air. .Thus,'the combusengine types, it has particular application to the turbo Air entrance section Compressor section Combustion section Turbine section Tailpipe section Accessory section Fuel system Starting system Ignition system 10. Cooling system 11. Lubrication system Of these sections, the present invention is primarily concerned with the combustion and turbine sections and atented Feb. 26, 1957 2 secondarily with the air entrance, compressor and tailpipe sections.

A turbo-jet aircraft engine operates in the following manner: Air flows into the engine through the air entrance section and then into the compressor section, where it is usually compressed to a pressure of about 45 to.180 p. s. i. g. The air entrance and compressor sections may follow any one of several designs; and the compressor section may possess either an axial or a centrifugal compressor. If. of the centrifugal type, the compressor may additionally possess either a single or a double entry.

From tbecornpressor section, the compressed air flows into the combustion section where it iscombined with a metered and atomized or prevaporized amount of fuel and its temperature increased by combustion of the fuel. It will be noted that the air flow in. this section is such that only a relatively small amount of the air actually mixes with the fuel at the point of combustion. This portion of the air is generally referred to as the primary air supply. The weight ratio of primary air to fuel is generally between 101:0 1 and 20 to 1. a 7

Following the combustion of the fuel, the combustion products are almost immediately and intimately mixed tion products are cooled from a combustion temperature of about 3500 to 4000 F. to an average temperature of. about 1400 F." The latter temperature isdictated at the present time by the types of metals and metal alloysthat are presently available for use within the turbine section of an engine. This limitation will be considered in greater detail when describing the'turbine section.

The combustion section may be any one of the conventional types as, for example, one that employs multiple combustion-chambers (cans) or one that uses an annular combustion liner or chamber (a burner basket).'

In the first'of these types, the air flow is split upon leaving the compressor and equal portions sent toeach can, where theseportions are combusted with portions of the fuel. The combustion products are then recombined with secondary air .and routed to the turbine section.

When the combustion section is of the burner basket type, the primary portion of the air is diverted from the main stream and directed toward the fuel injector within the basket where it burns with'the fuel. The remaining or secondary air is then mixed with the products of combustion at a point prior to their entrance into the turbine section. r a

It is apparent that a wide'range of temperatures usually exists within either of the abovetypes of combustion sections. As has been mentioned, the flame temperature is generally in the neighborhood of about 3500 to 4000 F., while the gases entering the turbine section are generally at about 1400" F. Since the air flowing into the combustion section may be at atmospheric temperature up to 300-600 F., it follows that certain parts of the combustion section are at considerably lower temperatures than the 1400 F..value. This condition has resulted in a very serious problem, namely the formation -of carbon and carbonaceous deposits within this particular section.

The problem of carbon deposition .within the combus tion section of a jet engine has been recognized bynthe art, but relatively little progress has been made. toward reaching its solution. Observation of a number of combustion sections now indicates that carbon deposition is most prevalent or most prone to occur-.011, those metal parts of a combustion section whose temperature is. about 600" to 900:F; and especially about 750 F. Accord-. .ingly it is a primary object of the present invention to provide for the elimination of these deposits from a jet engine without interfering materially with the physical design of the engine. This and other objects will become more apparent with the following description.

The turbine section of a jet engine may contain one or more turbine rotors and one or more stages. In addition, the turbine blades maybe of the impulse and/ or reaction types and may or may not be shrouded.

The gaseous products of combustion and the excess air entering the turbine section from the combustion section cause the turbine rotor or rotors to revolve and to drive the compressor in the compression section and also auxiliaryequipment such as fuel pumps, lube oil pumps, generators, etc. It will be noted that approximately 80% of the useful power generated by a turbo-jet engine is utilized by the turbine to drive the compressor and the various auxiliaries. The remaining power is available to propel the aircraft. 'In a turbo-prop engine, this remaining power is transmitted to and drives a propeller, while in a turbo-jet engine, it is generally realized in a form of a pure reaction force.

The gases leaving the turbine then flow into the tailpipe section from whence they vent to the atmosphere. The design of this section may varyconsiderably. For example, it may have either a single or double exit, and it may also be of the variable orifice or adjustable exhaust nozzle type. When the tailpipe is provided with an afterburner, it is essential that a variable exhaust opening be provided to adjust for both normal and afterburning combustion conditions.

The tailpipe section normally operatesat .temperatures of about 900 to 1400 F. Inasmuch as the temperatures in this section are generally lower than those existing in the combustion and turbine sections and since the mechanical clearances within this section are less critical, the materials of construction employed here need not be nearly as stable as those employed in the other two named sections.

A brief reference was made earlier to the use of afterburners and afterburning." It is well to point out at this time that these terms describe one method of augmenting the thrust normally developed by a jet engine. There are actually three methods that are conventionally employed for this purpose. They are as follows:

1. Afterburning, or tailpipe burning I 2. Air bleed-01f 3. The use ofpower augmentation liquids It will be notedthat all of these methodshave primary application to turbo-jet and turbo-prop types of jet engines.

The first of the above methods, namely afterburning, briefly consists of injecting and combusting a portion of fuel within the tailpipe section of an engine. This portion is over and above the fuel charge that is conventionally burned in the combustion section. In other words, the tailpipe section is operated in the same manner as a ram-jet, since the combustion products formed at this point exit directly to the atmosphere without passing through a turbine.

When afterburner nozzles are provided in the tailpipe section, a diffuser is usually placed between the turbine section and the nozzles. This device serves to redistribute the gas flow in the tailpipe and to promote better combustion of the fuel issuing from the afterburner nozzles.

The air bleed-off system of power augmentation, as applied to a turbo-jet engine, employs a secondary or auxiliary ram-jet engine in addition to the primary turbine engine. In this system, a portion of the air from the compressor is fed to the secondary engine and is combusted with fuel that is separately .supplied to this engine, and additional thrust is thereby obtained. Water or a mixture of water and alcohol may be injected into the primary engine .to replace the air that has been di- 4 engine.

verted into the secondary The purpose of the water or the water and alcohol mixture is to compensate for the'air being diverted from the primary engine and to restore the full thrust of this engine. Thus, in a sense, the air bleed-off method of power augmentation is a combination of the methods that employ afterburning and power augmentation liquids. The first of these two methods has already been described; the latter will be described briefly below.

Power augmentation liquids such as water and mixtures of water and alcohol are conventionally injected into the compressor inlet or into the combustion chamber of a jetengine. The alcohols conventionally employed for this purpose are the low molecular weight aliphatic alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, etc. When the jet engine is equipped with a centrifugal compressor, the thrust augmentation liquids are usually injected into the compressor; but, when the engine employs an axial compressor, they are generally injected directly into the combustion chamber. In either event, the liquids act to cool the gases entering the turbine section, thereby permitting more fuel than normal to be burned within the combustion section without exceeding the temperature limits imposed by the materials of construction in the turbine section. The total mass flow of gases and vapors passing through the jet engine is thereby greatly increased with the result that the power output and thrust of the engine are correspondingly increased.

' Due to the high temperatures prevailing within the combustion-, turbine-, and tailpipe-sections of a jet engine, it is mandatory that special materials possessing excellent stability toward heat, oxidation and corrosion be employed in the construction of these sections. Since the turbine section presently limits the temperature levels that may be utilized in a jet engine, this section is presently constructed of only the most stable metals and alloys. For example, the turbine disks, buckets, blades, etc. are generally made of alloys and alloy steels that contain large amounts of metals such as chromium, nickel, molybdenum, cobalt, titanium, and tungsten. The combustion section and the tailpipe section are also constructed of alloys and alloy steels, but the lower temperatures and larger mechanical clearances, etc. permitted in these sections allow engine builders to use much less expensive materials inthese sections. It is common practice in these sections to use high nickel-chromium steels such as columbium-stabilized 18-8 stainless steel.

The fact that the combustion-, turbine, and the tailpipe-sections of a jet engine are subjected to very rigorous operating conditions makes it imperative that any corrosion occasioned by the use of petroleum or other type fuels be minimized as much as possible. Accordingly, it is an additional object of the present invention to avoid subjecting these various sections to any extraordinary corrosive conditions while still preventing the deposition of carbon and carbonaceous materials therein. To achieve these objectives, it is proposed that certain mixtures of metallic oxides or of oxide-forming metal compositions be introduced within those sections of a jet engine in which carbonaceous deposits are prone to form. This may be done by (a) incorporating the metal oxides or oxide-forming compositions within the air flowing to the pertinent sections; (1)) directly injecting the oxides or the compositions in the form of finely-divided solids, either dry or in liquid suspension, into the desired sections; (0) incorporating hydrocarbon-soluble compositions of the metals within the hydrocarbon fuel that is being supplied to the engine; (d) incorporating soluble compositions of the metals within the power augmentation liquid that may be used in the engine. Of these various methods, the incorporation of hydrocarbon-soluble compositions of the metals within the fuel is preferred.

.In carrying out any of the methods just described, it

is essential that a combination of two types ofmetal compositions be employed. Afirst type must consist of an oxide .or an oxide-forming composition of an alkaline earth metal, while a second type must consist of an oxide or an oxide-forming composition of an amphoteric metal.

As described earlier the term alkaline earth metal as used herein includes calcium, barium and strontium; and the term amphoteric metal includes lead, zinc, manganese, chromium, vanadium, aluminum, bismuth and molybdenum. The term oxide-forming composition is intended to include any composition that is capable of forming an oxide of one of the types desired under the conditions prevailing within the combustion section of a jet engine. In other Words, it must be capable of frming an oxide in an oxidizing atmosphere at a temperature within the range of about 500 F. to 4000" F.

The combined amount of alkaline earth metaland amphoteric metal-compositions to be introduced within an engine for the purposes of the present invention may be such a to provide from about 0.00001 to 0.1 weight percent of the combined metals, based upon the amount of fuel charged to the engine. It will be noted that although the amounts of the metals to be added are here expressed as pure metals, the metals are actually present in the form of various compounds or compositions.

It is preferred that amounts of an alkaline earth metal composition and of an amphoteric metal composition be added during the operation of an engine to provide from 0.0001 to 0.1 weight percent of the combined metals, again expressed as the pure metals and based on the amount of fuel charged to the engine. Particularly preferred are amounts of the two types of metal compositions sufiicient to provide from 0.01 to 0.1 weight percent of the combined metals, based on the fuel charged.

It is preferred that the compositions of the present invention be substantially free of acidic components such as sulfur and the various halogens.

It is additionally desired that the alkaline earth metalcontaining composition and the amphoteric metal-containing composition be added in individual amounts, such that the atomic ratio of alkaline earth metal/amphoteric metal within the pertinent sections of a. jet engine be from about, 1/1 to /1 and especially about 3/1 to 5/1. It.

will be noted that the about 1 or more.

As stated earlier in this description, the sections of a jet engine within which carbonaceous deposits are most apt to form are those in which the atomized and/ or burning fuel contacts metals whose temperatures are about 750 F. This thermal condition exists primarily within the comatomic ratio may be as much as bustion section, especially where the secondary air makes 0 contact with hot combustion gases and fuel in the vicinity of the liner and parts of the nozzle.

It is well to point out that in those cases where the amphoteric metal employed is lead, vanadium, bismuth or molybdenum, the alkaline earth metal composition serves a second important function. In addition to coacting with the amphoteric metal to eliminate carbon deposition, the alkaline earth metal prevents corrosion of internal parts of a jet engine that operate at temperatures in excess of about 1400 F., particularly in the presence of the added amphoteric metal composition. 1 It. has been found that each of these particular amphoteric metals, their oxides,etc. are extremely corrosive toward the metals and alloys that are used in ,jet engine construction. Fortu nately, however, it'has now also been found that the corrosion resulting from this source may be substantially eliminated within the affected sections by maintaining an atomic ratio of alkalineearth metal/amphoteric metal of greater than 1/1 and preferably at least 3/1. In general, the atomic ratios need not exceed about 5/ 1, but they may extend up to about 10/ 1.

It is particularly preferred that certain combinations of barium and calcium oxidesor their compositions be caused by the amphoteric metals listed above (e. g., lead, vanadium, bismuth or molybdenum). For example, it has been found that mixtures of alkaline earth metaland amphoteric metal-compositions are markedly less corrosive when the alkaline earth metal component of the alkaline earth metal composition is composed of about two atoms of barium to one atom of calcium than in those cases where either barium or calcium is used alone. This point is of particular interest in instances where a leaded gasoline is employed as the fuelin a jet engine. The practice of using leaded gasolines as jet fuels is a common one aboard aircraft carriers Where, because of logistics, it is extremely desirable to keep on hand just one type of fuel that is suitable for both internal combustion and jet engines.

As stated earlier, a particularly attractive method of introducing the amphoteric and alkaline earth metal compositions of the present invention consists in incorporating hydrocarbon-soluble forms of these metals directly within the hydrocarbon fuel supplied to a jet engine. Before considering the chemical forms of the metals to be employed directly in a hydrocarbon fuel, it is considered advisable to first discuss and describe the hydrocarbon fuels themselves. In general, the hydrocarbon fuel to be satisfactory for use in a jet engine should possess a low freezing point; it should not form excessive coke upon combustion; it should burn with a stable flame; it should be non-corrosive and it should have a high B. t. u. content 1. Freezing point 40 C. max. 2. Kinematic viscosity 6 centistokes max.

18 C. 3. Flash point 38 C. min. 4. Sulfur content 0.20% by wt. max. 5. Corrosion Only slight discoloration of copper strip. 6. Gravity No specification. 7. Accelerated gum 20 mg./ ml. max. 8. Residue (air-jet method) s 10 mg./ 100 ml. max. 9. Aromatics content 20% max. 10. Water tolerance Substantially immiscible. 11. Distillation:

Percent of]? at 392 F 10% min. Final B. Pt., "F 572. Loss l /2% max. 12. Heatof combustion 18,300 B. t. u. 1b.,

min.

aircraft carriers. Typical inspections for an aviation gaso line may be found in ASTM Specification No. D9l05 1T. It will be especially noted that this specification calls for gasolines containing from 2 to 4.6 ml. of lead tetraethyl per gallon of gasoline. It will also be noted that the tetraethyl lead is added in the form of a fluid mixture containing not less than 61% by wt. of tetraethyl lead and sufficient ethylene dibromide to provide two bromine atoms per atom of lead. In general, an aviation gasoline may therefore contain up to about 3.5 grams of lead per gallon of gasoline.

Having described the hydrocarbon fuels that are particularly desirable for use in jet engine, suitable amphoteric metal and alkaline earth metal-compositions may beconsidered. g

In general, any hydrocarbon-soluble or hydrocarboninsoluble compositions of the amphoteric or alkaline earth metals .of the present invention may be employed with the hydrocarbon fuels. It is essential, however, that the compositions form or exist as the oxides of the two types of metals under the conditions existing within the combustion section of a jet engine. It is also essential that the insoluble compositions have a mean particle size less than 10 microns and preferably less than 1 micron; and that they be dispersed throughout the fuels in the form of stable suspensions. Suitable conventional suspending agents may be used for this purpose.

Suitable hydrocarbon-soluble forms of the amphoteric and/or alkaline earth metals that may be added directly to a jet fuel include the hydrocarbon-soluble metal salts of naphthenic acids, aliphatic acids, and aromatic acids. The naphthenic acids are particularly preferred, especially those having molecular Weights in the range of about 150 to 300. The aliphatic acids, preferably saturate-d, may contain from about 8 to 22 carbon atoms. Preferred aromatic acids are those containing from about 7 to 12 carbon atoms.

Other organ'o-metallic compounds suitable for use in the practice of this invention include the hydrocarbonsoluble nlkyl, carbonyl, aryl, and alcoholate compounds of the various metals. In general, it is preferred that these compounds be readily combustible to the metal oxide or carbonate under the conditions prevailing Within the combustion section of a jet engine and that they have boiling points not in excess of about 600 F.

Another method for practicing the present invention concerns the direct injection of solid or liquid compositions of the two types of metal compositions directly within the combustion and/or turbine sections of a jet engine. Once again, the choice of compounds to be employed in carrying out this method should be made from the classes of compounds that are readily converted to the metal oxide or carbonate under the conditions prevailing within the pertinent engine sections. It is apparent that the metal oxides themselves may be used for this purpose.

A particularly attractive method for introducing the desired metal compositions Within a jet engine consists in dissolving appropriate forms of the metals within the water or Water-alcohol mixtures that are employed as thrust augmentation liquids within certain types of engines. For example, it is contemplated that the various metallic acetates, formates, lactates, etc. may be employed in this manner.

Insoluble forms of the two types of metal compositions may also be incorporated within thrust augmentation liquids when these liquids are employed in a jet engine. Once again the compositions must be of a type that may be converted to the corresponding metal oxides under the conditions prevailing within the combustion section of a jet engine. It is preferred that the insoluble compositions be in a finely-divided form and that they have'a particle size of less than about 10 microns and preferably less than 1 micron. It is additionally preferred that the compositions be dispersed throughout the thrust augmentation liquids as by mechanical agitation or by the. use of conventional dispersant additives. Particularly suitable com-positions include the oxides, hydroxides, hydrated or hydrous oxides and carbonates of the various metals. It will be appreciated that insoluble and soluble compositions may be employed together.

it will be noted that the alcohols generally employed in thrust augmentation liquids are the aliphatic alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, and the like.

To illustrate the effectiveness of the present invention, a number of laboratory tests were run in which mixtures ofcarbon and various; combinations of compositions containing the two types of metals were placed in an oven at 750 F. for 48 hours. This temperature was chosen because combustor data indicate this to be the most severe temperature for carbon formation. At temperatures appreciably above 750 F. the carbon oxidizes from air itself very rapidly, while at lower temperatures the carbon does not form. The weight percentage of carbon in each sample was determined before placing the samples in the oven and then again after the 43-hour period. The following table lists the results of these tests and shows their eifectiveness in eliminating carbon under these conditrons.

TABLE I Carbon removal from mixtures of carbon and metal composmons [Oonditionsz 750F., 48 hrs] Mols M01 Ratio, Carbon Sample Additive Additive Additive] Loss, X 10 Carbon Wt.

(percent) 1 None 44. 0 2 0:10 2. 50 0. 02 9s. 4

PhD 0. 83 3 {B5003 2. 50 0.02 92.1

Pb 0 0. 83 4 {C80 2.50 0.02 88.0

V205 0. 83 5 BZLCOz 2. 50 0. 02 95. 4

V20 5 0. 83 6 CaO 2. 50 0. 02 97. 5

M003 0. 83 7 BaCOs 2. 50 0. 02 0s. a

M003 0. 83 8 Ci-LO 2. 50 0. 02 79. 4

It will be notedthat the metallic compositions greatly increased the amount of carbon lost from the samples, although at this high temperature some carbon is lost solely because of direct air oxidation. This fact is shown by the results in which carbon was run in the absence of any metallic additive.

The above data clearly present the basic concept of the present invention. As explained earlier, the concept involved herein consists essentially in laying down a mixture of metal oxides in intimate contact with the carbon formed in a jet engine. The oxides need not necessarily come. from combustion of an additive in the fuel itself. They can also originate from the combustion of a salt or compound added in the thrust augmentation fluid or by any of the other techniques described earlier. Direct addition of the metal compounds to the fuel, however, is generally preferred.

Another series of laboratory tests were conducted to determine the corrosiveness of the various amphoteric metals toward the type of materials that are conventionally employed in the construction of the internal parts of a jet engine. In this series of tests, samples of type SAE 30304 stainless steel were immersed in crucibles containing oxides of the various amphoteric metals. Another set of samples of'a second metal, Refract alloy 80, was placed in another set of similar crucibles containing the same metallic oxides. The crucibles were then placed in a muflle furnace for 48 hours at 1650" F. and air was circulated through the furnace at a rate suflicient to change the atmosphere in the furnace times each hour. Following the 48-hour period, the crucibles were withdrawn from the furnace, the metallic samples were electrolytically descaled in molten caustic and then weighed. The lossin weight of each metal sample was then determined. The results of this series of tests are 7 given in the following table. a

h TABLEII Efiect of metal oxides on corrosion of jet engine metals '[Testperlom 48 hrs. 1650F.]

Metal oxide Analysis: 18% Or, 8% Ni (type SAE 30304).

Analysis: 30% oo, 20% Ni, 20% Or, 10% Mo, 5% W, 13.5% Fe, 0.5% Mn, 0.11% o, 0.05% Si.

From the data in the above table, itmay be seen that lead,vanadium, bismuth and molybdenum oxides are extremely corrosive toward the metals and alloys generally employed in jetengines. From these data, it is readily apparent that an alkaline earth metal oxide must be added along with these particular 'amphoteric metal oxides not only for the purpose of eliminating-carbon deposits but also to prevent these particular amphoteric oxides from corroding the equipment. Theetfectiveness of alkaline earth metal oxides such as barium and calcium in preventing such corrosion is well-illustrated in Table III below. In this instance, simple strips of type SAE 30304 stainless steel were immersed in crucibles containing mixtures of barium or calcium compositions and an amphoten'c metal oxide. The samples and the crucibles were handled according to the same procedure as was employed in obtaining the data given in Table II above.

TABLE III Corrosiveness of mixtures of alkaline earth metal compounds and amphoteric metal compounds [Conditions: 1650 F., 48 hrs., type SAE 30304 (18-8) stainless steeL] 1 M01 ratio of alkaline earth metal compound to amphoterlc meta compound.

The data in Tables H and III above clearly demonstrate the fact that alkaline earth metal compositions in admixture with some amphoteric metal compositions are markedly less corrosive than the amphoteric metals alone. This improvement is particularly true for lead, bismuth, vanadium and molybdenum. The results in Table III further indicate that the corrosiveness of the mixtures is particularly reduced when the mol ratio of the alkaline earth metal compound to the amphoteric metal compound is at least about 3/1.

Another series of tests were made to determine the eiIect of employing more than one alkaline earth metal composition in conjunction with an amphoteric metal composition. Accordingly, sample strips of type SAE 30304 stainless steel were immersed in crucibles containing mixtures of barium and/ or calcium compositions and an amphoteric metal oxide, namely lead oxide. The samples and the crucibles were handled according to the same procedure as was employed in obtaining the data in Tables II and HI.

TABLE IV Barium and Calcium Content Weight loss expressed as atoms/atom of Pb 01' stainless Absolute Sample No. strip Relative Wt. Loss, 5 to PhD Alone Percent Percent ,Wt. Loss of Specimen Barium Calcium Ba+0a 18-8 Stainless Relractalloy 10. 0 Steel 1 80 1/1 168 16.8 3/1 10. 5 1. 1 5/1 6 0. 6 1/1 14. 0 3/1 6. 4, 4. 9 5/1 20 2.0 3/1 11. 5 1. 2 3/1 7 0. 7 3/1 52 5. 2 3/1 73 7. 3

1 No barium or calcium added.

As may be seen above, lead oxide by itself causes a stainless steel strip to lose 10% of its weight through corrosion of the strip. It may further be seen that the addition of barium oxide or calcium oxide serves to pro tect the steel strips against lead corrosion providing the calcium or barium is present in amounts in excess of one atom of calcium per atom of lead. It will be noted that where one atom of barium or calcium per atom of lead is employed, the degree of corrosion is actually worse than the case Where neither barium nor calcium is present at all.

Where the barium or calcium is present in the amounts of three or more atoms of barium per atom of lead, the corrosion of the steel strips is very materially reduced.

It will be particularly noted that barium and calcium in admixture are markedly more eifective in preventing the corrosion of stainless steel strips than is barium or calcium alone. For example, where 2 atoms of barium and 1 atom of calcium are present per atom of lead, the stainless steel panels are corroded less than where 3 atoms of either barium or calcium alone are present.

While the data in Table III were obtained using only lead oxide, it is contemplated that the same phenomenon demonstrated there would take place if molybdenum oxide, bismuth oxide or vanadium oxide were substituted for the lead oxide. Thus, the carbon-removing qualities of these particular amphoteric metals can be completely utilized without fear of introducing even more serious corrosion problems.

This corrosion-inhibiting synergism demonstrated by mixtures of calcium and barium can be especially utilized in connection with leaded aviation gasolines when they are employed as fuels for jet engines. When the amount of lead (present generally in the form of a lead alkyl) is insuflicient to promote the desired degree of carbon removal, one of the amphoteric metal compositions as well as one or more of the alkaline earth metal compositions may be added to the gasoline. The compositions in this instance are preferably soluble in the gasoline.

It will be appreciated that many other materials may be added to the fuel compositions of the present invention without departing from the scope or spirit of the invention. For example, ignition promoters such as amyl nitrate, scavenging agents such as ethylene dibromide and ethylene dichloride, corrosion inhibitors, oxidation inhibitors and the like may also be employed.

What is claimed is:

1. The method of eliminating carbonaceous deposits within the combustion section of a jet-type aircraft engine operating on a hydrocarbon type fuel which consists of introducing within said combustion section both an alkaline earth metal compound and an amphoteric metal compound in amounts sulficient to furnish a total of the said metals of from 0.00001 to 0.1 weight percent of said fuel, sai'd alkaline earth metal compound being selected from the class consisting of barium compounds and mixtures of barium compounds and calcium compounds in which mixtures barium atoms predominate numerically over calcium atoms, the atomic ratio of total alkaline earth metal to totalamphoteric metal introduced into the. said combustion section being in the range of from 3 to 1 to 5 to 1, said metal compounds being of a character to form oxides of the said amphoteric metals and the said alkaline earth metals in an oxidizing atmosphere at temperatures of from 500 F. to 4000 F.

2. Method as defined by claim 1 in which the total of said metals constitutes from about 0.01 to about 0.1 weight percent of said fuel.

3. Method as defined by claim 1 in which the amphoteric metal is lead and the alkaline earth metal is barium.

4. Method as defined by claim 1 in which the amphoteric metal is molybdenum and the alkaline earth metal is barium.

5. Method as defined by claim 1 in which the amphoteric metal is vanadium and the alkaline earth metal is barium.

6. Method as defined by claim 1 in which the amphoteric metal is lead and in which the alkaline earth metals consist of barium and calcium in the proportion of two atoms of barium and one atom of calcium per atom of lead.

7. A clean-burning fuel composition, substantially noncorrosive at temperatures in excess of about 1400 R, which consists essentially of a hydrocarbon mixture, adapted to be burned within a jet-type aircraft engine, to which have been added a hydrocarbon-soluble compound of an alkaline earth metal and a hydrocarbon-soluble compound of an amphoteric metal, said compounds being present in amounts to furnish a combined weight of said metals of from about 0.00001 to about 0.1 percent of the Weight of said hydrocarbon mixture, said alkaline earth metal compound being selected from the class consisting of barium compounds and mixtures of barium compounds and calcium compounds in which mixtures the number of barium atoms is greater than the number of calcium atoms, the atomic ratio of total alkaline earth metal to total amphoteric metal in" the' said composition being' in the range of from 3 to 1 to- 5 to 1, said metal compounds being of a character to form oxides of the said amphoteric metals and the said alkaline earth metals in an oxidizing atmosphere at temperatures of from 500 F. to 4000* F. 8. Fuel composition as defined by claim 7 in which the metal compounds are metal salts of naphthenic acids hav- References Cited in'the'file of this patent UNITED STATES PATENTS 2,151,432 Lyons et al Mar. 21, 1939 2,230,642 Fischer et al Feb. 4, 1941 2,338,578- Downing et a1. Jan. 4, 1944 2,560,542 Bartleson-etal July'17,-1951 2,671,758 Vinograd et al Mar. 9, 1954 2,673,145 Chandler Mar. 23, 1954 2,706,149 Brennem-an Apr. 12, 1955 OTHER REFERENCES Journal of the American Rocket Society, No. 72 (December 1947), p. 17.

The American Bulletin of Interplanetary'Society, No. 16'

(February 1932), pp. 8-10.

Richmond Times-Dispatch (Jan. 12, 1947), p. 18. 

1. THE METHOD OF ELIMINAING CARBONACEOUS DEPOSITS WITHIN THE COMBUSTION SECTION OF A JET-TYPE AIRCRAFT ENGINE OPERATING ON AHYDROCARBON TYPE FUEL WHICH CONSISTS OF INTRODUCING WITHIN SAID COMBUSTION SECTION BOTH AN ALKALINE EARTH METAL COMPOUND AND AN AMPHOTERIC METAL COMPOUND IN AMOUNTS SUFFICIENT TO FURNISH A TOTAL OF THE SAID METALS OF FROM 0.00001 TO 0.1 WEIGHT PERCENT OF SAID FUEL, SAID ALKALINE EARTH METAL COMPOUND BEING SELECTED FROM THE CLASS CONSISTING OF BARIUM COMPOUNDS AND MIXTURES OF BARIUM COMPOUNDS AND CALCIUM COMPOUNDS IN WHICH MIXTURES BARIUM ATOMS PREDOMINATE NUMERICALLY OVER CALCIUM ATOMS, THE ATOMIC RATIO OF TOTAL ALKALINE EARTH METAL TO TOTAL AMPHOTERIC METAL INTRODUCED INTO THE SAID COMBUSTION SECTION BEING IN THE RANGE OF FROM 3 TO 1 TO 5 TO 1, SAID METAL COMPOUNDS BEING OF A CHARACTER TO FORM OXIDES OF THE SAID AMPHOTERIC METALS AND THE SAID ALKALINE EARTH METALS IN AN OXIDIZING ATMOSPHERE AT TEMPERATURES OF FROM 500* F. TO 4000* F. 